The invention relates to a fuel cell battery comprising a stack of planar cells and to a use of the battery in accordance with the invention.
A centrally symmetric fuel cell battery is known from EP-A-0 473 540, the interconnectors of which are designed as special temperature equalization bodies. These equalization bodies are heat exchangers, by means of which reaction heat is transferred to supplied air before the latter is brought into contact with the PENs, the electrochemically active elements of the fuel cells (PEN: Positive electrode, solid Electrolyte, Negative electrode). This heat exchanger is a plate-like hollow body, in the inner space of which the heat transfer to the air takes place. The two outer side surfaces of the interconnector have profilings, by means of which on the one hand electrical contacts to the electrodes of the PENs are produced and which on the other hand leave gap-like electrode spaces free between the interconnectors and the electrodes for the reaction components (air, fuel gas).
During the operation of the battery, temperature gradients arise in the PENs which are directed radially. These gradients are relatively small so that thermal stresses cause no damage, in particular tears, in the sensitive solid electrolytes of the PENs.
The interconnectors, which are designed as heat exchangers, are expensive; their share of the costs in the manufacture of the fuel cell battery is considerable. A plurality of suggestions for the manufacture of interconnectors have already been made with the goal of reducing the costs. For example in EP-A-0 936 688 an interconnector designed as a heat exchanger is described, for the manufacture of which one or two sintered bodies are used, with the sintered bodies being pre-shaped, namely through pressing of a powder mixture into the shape of the finished part and subsequent sintering.
It is an object of the invention to create a fuel cell battery, the manufacturing costs of which are further reduced.
The fuel cell battery, which contains a stack of planar cells, has the following features:
a) Electrochemically active plates, the so-called PENs, and interconnectors are arranged in an alternating sequence.
b) The PENs and accordingly the interconnectors have in each case a first edge and a second edge, between which a straight or curved zone with a largely constant width extends.
c) This zone is subdividable into sectors through which the two edges are connected.
d) The interconnectors have profilings by means of which two fluids can be separately conducted through the cells.
e) In each sector there are provided entry points for the first fluid at the first edge, entry points for the second fluid at the second edge as well as outlet points for both fluids.
f) The outlet points open into a common passage.
g) The second fluid is provided as a heat carrier medium for reaction heat which is liberated at the PEN during an operation.
h) In accordance with the invention the interconnectors are single layered and their profiling is in each case executed in such a manner that both fluids are always conducted in contact with the PEN.
i) A turning region is provided at the first edge by means of which a reversal of the flow direction of the second fluid results.
j) Furthermore, the dimensioning of the cells is executed in such a manner that harmful thermal stresses in the PEN are not exceeded during the operation.
The invention is based on a recognition relating to the following considerations. As model calculations have shown, the results of which are illustrated in the named EP-A-0 473 540, the reaction heat which is liberated at the PENs is transferred to the interconnectors mainly through thermal radiation. The heat transport as a result of a heat conduction through the air-filled electrode space is practically negligible. The wall of the interconnector has at each point a temperature which differs by only a few degrees Kelvin from the temperature at the corresponding point of the PEN. This wall temperature thus does not depend significantly on the temperature of the air to be heated up in the hollow space. The reason for this is a relatively low heat flow between the wall and the air.
The fact that such conditions are present in the heat transport in the interconnectors is not obvious as a result of the disclosed results of the model calculation. If one however becomes conscious of these conditions, then one can pose the question whether an interconnector which is designed as a hollow body is actually necessary in order to conduct off the reaction heat in the manner which is described in EP-A-0 473 540. The answer, that it is possible in a different manner, is given by the solution in accordance with the invention. The air (the second fluid) can already be brought into contact with the PEN at the entry point so that it is possible to design the interconnector in a single layerxe2x80x94thus more economicallyxe2x80x94and not as a double layered hollow body (two walls, one hollow space). The air to be heated admittedly causes additional temperature gradients in the PEN. Such gradients, which are directed tangentially (azimuthally), have an order of magnitude similar to that of the radial temperature gradients which result in the operation of known cells. Thus it can be expected that the thermal stresses do not take on substantially greater values in the use of the single layer interconnectors. Measures can also be provided as a result of which the additional components of the temperature gradients can be kept relatively low.
The named prior art relates to centrally symmetrical fuel cell batteries. The solution in accordance with the invention can however also be applied to batteries with for example rectangular cells in which the fuel gas (first fluid) is fed in at the one side and the air (second fluid) at the opposite side. In addition it is the case that the first fluid can be any desired gas composition with combustible components and the second fluid is a gas containing oxygen, under the assumption that exothermic reactions which supply electrical current can be carried out at the PEN with these gases.
The interconnectors of the fuel cell battery in accordance with the invention yield further advantages thanks to their being single layered:
a) the mass of the battery is smaller than that of the known batteries;
b) the constructional size is also reduced. Therefore the battery in accordance with the invention can also be easily used for a mobile application, for example as current supplying components in an automobile or for an emergency power aggregate which is to be rapidly transportable to a potential site of use.